1. Field of the Invention
The present invention relates to a display device having a diffraction grating to separate an incident light into color light components to establish color separation and a transmission type light valve such as liquid crystal panel to receive the diffracted light for realizing a color display. The present invention also relates to a display device having a light source adapted to provide parallel light beams to a diffraction grating. The present invention also relates to a display device having a diffraction grating to separate an incident light into color light components and a polarization separation means enabling the separated light to be used.
2. Description of the Related Art
A color display device comprises a light valve such as a liquid crystal panel which has a color filter having red, green and blue portions. The display device also has display electrodes corresponding to the red, green and blue portions of the color filter, so that a light passing through a certain color portion passes through the corresponding display electrode or is blocked by the corresponding display electrode by controlling a voltage on or off. The color filter allows a light component having a particular color bandwidth of wavelength to pass therethrough and does not allow the remaining light components therethrough, the remaining light components being reflected or absorbed. Therefore, a viewer can see the transmitted light component having a particular color bandwidth or wavelength. A problem in this type of color display device resides in that a portion of the light which passes through the color filter is used but the remaining portion of the light is not used.
A color display device using a diffraction grating, in place of a color filter, is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 62-293222 and No. 62-293223.
The diffraction grating separates an incident light such as natural light into a diffracted light of a zeroth order which is not substantially diffracted, a diffracted light of first order having different color light components having respective color bandwidths or wavelengths, a diffracted light of second order and so on. If the diffracted light of zeroth order and diffracted light of first order can be effectively used, it is expected that efficiency of available light can be increased, compared with the case using the color filter. However, it is difficult to match the diffracted light of zeroth order and the diffracted light of first order with the pitch of the display electrodes, since the diffracted light of zeroth order and the diffracted light of first order emerge from the diffraction grating at different angles. If a portion of the diffracted light of zeroth order and the diffracted light of first order is not used for display and emerges from the display device in an uncontrolled manner, an image with high quality cannot be realized.
In the above described Japanese Unexamined Patent Publication (Kokai) No. 62-293222, the display electrodes of the liquid crystal panel are formed so that light components having red, green and blue bandwidth or wavelength of the diffracted light of first order pass through the display electrodes and the diffracted light of zeroth order is directed to a shading portion of the liquid crystal panel. In this case, the shading portion is arranged at a position between the red, green and blue display electrodes of one picture element or at a position just adjacent to the red, green and blue display electrodes of one picture element.
Therefore, the diffracted light of zeroth order and the diffracted light of first order are made incident to a region within one picture element. However, it is difficult to arrange that the diffracted light of zeroth order and the diffracted light of first order impinge against the surface of the liquid crystal panel at a constant pitch, since the angle between the diffracted light of zeroth order and the diffracted light of first order is different from angles between the light components of the diffracted light of first order. Therefore, it is difficult to form the respective color display electrodes of one picture element so that the display electrodes have an identical shape and are arranged at a constant pitch. Therefore, it is difficult to form many, small picture elements within a certain display area. In addition, if the diffracted light of first order on the .+-. sides is to be used, the difference of the incident angle to the liquid crystal panel becomes larger.
In the above described Japanese Unexamined Patent Publication (Kokai) No. 62-293223, light components having red and blue bandwidths or wavelength of the diffracted light of first order passing through the diffraction grating are used but the light component having the green bandwidth of wavelength of the diffracted light of first order is not used; The diffracted light of zeroth order is used to realize a green color. However, the incident light must include a large amount of the light component having the green bandwidth or wavelength since the diffracted light of zeroth order is substantially identical to the incident light made incident to the diffraction grating, so the amounts of the light components having red and blue bandwidths or wavelength of the diffracted light of first order is reduced.
Also, in the case too, it is difficult for the diffracted light of the zeroth order and the diffracted light of the first order to impinge against the surface of the liquid crystal panel at a constant pitch, since the angle between the diffracted light of zeroth order and the diffracted light of first order is different from angles between the light components of the diffracted light of first order. Therefore, it is difficult to obtain a display device with high definition. In this case, particularly, the light component having the green bandwidth or wavelength of the diffracted light is shaded, but this light component is located at the center of the region in which the diffracted light of first order is distributed. Therefore, the shading portion must be provided in a narrow region between the red and blue display electrodes.
In the display device having the diffraction grating to separate an incident light into color light components, it is desirable that light beams are as parallel as possible to each other and parallel beams are made incident to the diffraction grating.
In the liquid crystal display device, a polarizer and an analyzer are usually used with a liquid source, and generates heat by absorbing the light. Particularly, in the projection type display device, it is necessary to use a strong light source, so the amount of heat generated by the polarizer increases and it is necessary to cool the polarizer.
The polarizer absorbs approximately one half of the light from the source, and the absorbed light is not used, so the efficiency of the available light is reduced. Therefore, means for improving the efficiency of the available light is required. For example, Japanese Unexamined Patent Publication (Kokai) No. 6-324329 discloses a liquid crystal display device having a polarization beam splitter and an array of micro-lenses. According to this arrangement, it is possible to use both the P-polarized light and the S-polarized separated by the polarization beam splitter. However, it is further desired to more and more effectively use the separated P- and S-polarized lights.